1. Field of the Invention
The present invention relates to a method and apparatus for recovering the estimated velocity of a mobile station in a mobile communication system.
2. Description of the Related Art
In next generation wireless communication systems, resource allocation based on accurate channel information plays an important role in supporting large-capacity multimedia packet services with limited frequency resources. The moving velocity and velocity information of a mobile station are important factors of channel information for such resource allocation, and are utilized as important information for power and handoff control in a cellular communication system.
More specially, the moving velocity or Doppler information of a mobile station may be used in various applications. For example, the moving velocity or Doppler information of a mobile station is used as coherent time information for channel estimation or as feedback information for beam-forming or channel precoding. Also, the moving velocity or Doppler information of a mobile station may be used as a measurement for effective resource allocation aimed at increasing available resources within a cell.
A mobile station experiences a fading effect and Doppler shift effect of a received signal. In a mobile communication system, the power of a transient signal received at an antenna of a receiver generally corresponds to a sum of signals received through multi-paths that are the result of scattering and reflecting of a signal transmitted from a transmitter to the receiver, and the received signals may be roughly divided into two components, that is, slow fading and fast fading. The slow fading is caused by terrain topography between a transmitter and a receiver, and receive power varies according to measurement locations. The fast fading is also referred to as “Rayleigh fading”, and is caused by scattering and reflecting of a signal, resulting from obstacles on a transmission path, such as buildings, trees, and vehicles. In this way, due to the influences of the slow fading and the fast fading, the power of a signal received by a mobile station varies every moment.
The Doppler shift effect generates a frequency error in a received signal in proportion to the moving velocity of a mobile station with respect to a base station.
Thus, using this characteristic in which a frequency error in a received signal due to the Doppler shift effect is proportional to the moving velocity of a mobile station, it is possible to estimate the velocity of the mobile station. In other words, the velocity of the mobile station may be estimated by detecting the maximum Doppler frequency, that is, the Doppler shift, of a signal received from a base station in a mobile communication system.
The above-mentioned moving velocity or Doppler information of a mobile station may be represented by a typical Doppler power spectrum as illustrated in FIG. 1.
FIG. 1 is a graph illustrating a Doppler power spectrum of the related art.
Referring to FIG. 1, a Power Spectral Density (PSD) in the frequency domain is shown for a mobile station. The PSD is plotted within a range between two frequencies that are equidistant (the same frequency distance fm) left and right from the center frequency fc, that is, a range from fc+fm to fc−fm. Herein, the same frequency distance fm will be referred to as the “maximum Doppler shift”. It can be noted that the PSD rapidly increases as the frequency becomes more distant from the center frequency in the left and right direction.
The PSD in the frequency domain, S(f), is expressed by the following equation:
                              S          ⁡                      (            f            )                          =                  1.5                      π            ⁢                                                  ⁢                          f              m                        ⁢                                          1                -                                                      [                                                                  f                        -                                                  f                          c                                                                                            f                        m                                                              ]                                    2                                                                                        (        1        )            
The PSD in the frequency domain can also be calculated as a frequency-domain value by using a channel impulse response as given in the following equation:S(f)=FFT{Rhk(−τ)}=FFT{h*(−τ)h(τ)}=|H(f)|2  (2)
where, Rhk(τ) denotes the auto-correlation value of a corresponding channel, h(τ) and H(f) denote channel responses in the time domain and the frequency domain respectively, {circle around (x)} denotes the convolution operator, and FFT denotes the fast Fourier transform.
By obtaining a value of the maximum Doppler shift through Equations (1) and (2), the velocity of a mobile station can be calculated as given in the following equation:
                    v        =                              c            ·                          f              m                                            f            c                                              (        3        )            
where, c is the velocity of light.
In order to calculate the velocity of a mobile station, the mobile station includes a Doppler shift calculating unit configured as illustrated in FIG. 2.
FIG. 2 illustrates a Doppler shift calculating unit provided in a mobile station of the related art.
Referring to FIG. 2, the Doppler shift calculating unit 200 includes a channel impulse response calculator 202, a summator 204, a Fast Fourier Transformer (FFT) 206, a PSD calculator 208, a PSD peak searcher 210, and a Doppler/velocity calculator 212.
The channel impulse response calculator 202 obtains channel impulse response h(t) values according to multi-paths from a channel estimator (not shown), and delivers the obtained values to the summator 204. Here, h(t) is the channel impulse response of a received preamble signal.
The summator 204 obtains a summation value by summating the respective channel impulse response values obtained according to the multi-paths, and delivers the obtained summation value to the FFT 206. If respective PSDs are calculated for the multi-paths, then the summation operation of the summator 204 may be omitted.
The FFT 206 calculates H(f) values by performing a Fast Fourier Transform (FFT) operation for channel impulse response values at respective predetermined sampling points, and delivers the calculated H(f) values to the PSD calculator 208. The PSD calculator 208 calculates PSD values by squaring the respective H(f) values, the number of which corresponds to that of the sampling points, and delivers the calculated PSD values to the PSD peak searcher 210.
The PSD searcher 210 searches for a Doppler shift index having the maximum PSD value among the PSD values, the number of which corresponds to that of the sampling points, and converts the Doppler shift corresponding to the searched Doppler shift index into a Doppler shift/velocity value. The Doppler shift index will be described in more detail below.
FIG. 3 illustrates a result of a simulation according to a method of the related art for estimating the velocity of a mobile station.
Referring to FIG. 3, the x-axis represents a true velocity, and the y-axis represents an estimated velocity. As an example, when a true velocity is equal to or greater than about 40 km/h, aliasing occurs due to an insufficient sampling rate. Thus, since the estimated velocity corresponding to a true velocity of 40 km/h or greater does not reach the actual velocity, the graph shows a folded shape at certain points (i.e., folding effect).
Generally, in order to calculate a PSD, a channel impulse response is obtained by estimating the channel of a preamble signal allocated in units of frames. As an example, the allocation period of the preamble signal is 5 msec as specified in the Institute of Electrical and Electronics Engineers (IEEE) standards 802.16e or 802.16m.
In this case, if the sampling theory defined by the following equation is applied, aliasing occurs when the Doppler shift has a value of 500 Hz or greater at a period of 5 msec:
                              f          s                =                              1                          T              s                                ≥                      2            ·                          f              d                                                          (        4        )            
where, fs denotes a sampling frequency, Ts denotes a sampling period, and fd denotes the Doppler shift.
That is, when the Doppler shift cannot satisfy the above condition as given in Equation (4), there is a problem in that the folding effect is caused by aliasing.